Conversion of wave motion into electrical energy



. Dec. 22, 1942. B'. B. BAUER ,305, 7

CONVERSION OF WAVE MOTIONINTQ ELECTRICAL ENERGY Filed Ap ril'7, 1941 e Sheets-Sheet 1 o i a 0 .99 26k 4 +160 E Z Z 8 3 I I la Z0 E M C 2 I Dec 22, 1942. v B U R 2,305,597

' CONVERSION OF WAVEMOTION INTO ELECTRICAL ENERGY Filed April 7, 1941 6 Sheets-Sheet 3 j INVENTOR.

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B. B. BAUER Dec. 22, 1942.

' CONVERSION OF WAVE MOTION INTO ELECTRICAL ENERGY 6 Sheets-Sheet 4 Fild April 7, 1941,

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W. NNH i ll INVENTOR. Z3024 K. km

Dec. 22, 1942. a. B. BAUER couvsasxou 0F WAVE MOTION mwo ELECTRICAL ENERGY Filed April 7. 1941 6 Sheets-Sheet 5 I 0211MB Baum/' Dec. 22,1942. t ,9, BAUER 2,305,597

CONVERSION OF WAVE MOTION INTO ELECTRICAL ENERGY Filed April 7, 1941 e Sheets-Sheet e Patented 22,1942

PATENT QFFICE CONVERSION or WAVE MOTION nv'ro ELECTRICAL smo Benjamin B. Bauer, Chicago, 111., assignor to S. N. Shore and Frances Share, trustees, doin; business as Shore Brothers, a'partnership Application April '2, 1941, Serial No. 387,218

U 15 Claims.

4 l This invention relates to apparatus for conversion of wave motion into electrical energy and the converse. More particularly it relates to instruments of unidirectional nature, i. e., in which the instrument is active preferentially in one direction only, throughout an extensive range of frequencies, being relatively inoperative in other directions. This application constitutes 'a, continuation-impart of 'my co-pending. application, Serial No. 232,439 for Conversion of wave motion into electrical energy, etc., now Patent No. 2,237,298. Other copending applications which are also continuations-in-part of Serial No. 232,439 are Serial No. 387,215 entitled "Conversion ofwave motion intoelectrical energy, etc.," filed April 7, 1941; Serial No. 387,217 entitled Conversion of wave motion into electrical energy, etc.," filed April 7, 1941; and Serial No. 387,438 entitled Conversion of wave motioninto electrical energy, etc., filed April 8, 1941.

Unidirectional operation has previously been obtained in both the transmitting and receiving .transducers through a' combination of a unit having a nondirectional (circular) polar sensitivity pattern with one having a" bidirectional 25 (cosine-law) polar sensitivity pattern. A combination of twosuch units causes the resulting polar sensitivity pattern to be unidirectional- (cardioid) in shape, and it has been applied extensively in the past to transmitting antennas, microphone-apparatus, etc. For this latter application, one of the units is commonly made to operate on the, pressure component of the sound wave (pressure transducer) and theother upon the pressure-diiierence of the sound wave'35 Addition or cancellation (velocity transducer). of the voltages generated in each unit occurs depending upon whether the incidence of sound is from the front incidence) or irom the rear obviously; 40 view, the section being taken as indicated at line (180 incidence) of the instrument. the voltages generated by both units for the 180 incidence should be substantially equal and opposite in phase throughout the frequency range ,in which the cancellation is desired, which because of inherent diflerences in construction and operating principle is a diflicult' thing to obtain in microphones operatingupon dissimilar vcomponents of the sound wave.

One important object of my invention is .to 'provide a unidirectional transducer operating over a'wide frequency range and comprising in part two transducing elements operating on the 7 a same component of the sound wave, thus doing away with the necessity or subtracting outputs 5 this specification proceeds.

Figure '1 is a diagrammatic layout of generalized apparatus embodying my invention; Fig. 2,

a vector diagram showing the voltage relation ships for a zero degree incidence of sound; Fig.

3, a similar view to Fig. 2, but representing the 180 incidence of sound; Fig. 4, a diagrammatic view of a specific embodiment comprehended within the diagram of Fig. 1; Fig. 5, a polar diagram illustrating the directional characteristics of the transducer of Fig. 4;'Fig. 6, a diagrammatic and sectional viewoi" a unidirectional crystalmicrophone; Fig. 7, a rear view in elevation of the same; Fig. 8, ari equivalent. electrical circuit of the microphone-shown in Fig. 6; Fig.

9, a frequency response curve of the microphone 'shown in Fig. 6, the upper curve showing the front side response and the lower dotted line showing the decrease of response for the rear incidence sound; Fig: 10, a part sectional view of a unidirectional dynamic microphone; Fig. 11, a front view of the same; Fig. 12, a diagrammaticview of the equivalent electrical circuit of the microphone shown in Fig. 10; Fig. 13, a cross sectional view of the unidirectional crystal microphone equipped with an acoustical resistance formed of cloth; Fig. 14, a front elevation of a unidirectional moving conductor microphone; Fig. 15, a sectional view, .the section being taken as indicated at line 15 of Fig. 14; Fig. 16, a rear elevation of the structure shown in Figm 14 Fig.

17, a diagrammatic view of the equivalent electrical circuit of the microphone shown in Fig. 14;

Fig. 18, abroken front elevation of another modiflcation of the invention; Fig. 19, a sectional IQ of Fig. .18; Fig. 20, a sectional view, the section being takenas; indicated at line 20 of Fig. 18: Fig. 21, a diagrammatic view of the equivalent electrical circuit of the device shown in Figs. 18

to20; and Fig. 22, a diagrammatic view showing various polar patterns obtained according to the invention. 1 My, invention is principally 'applicableto pro- -ductionand reception of sound waves in air,

' although it will become app rent to those skilled in the art thatfit may be equally applicable to wave phenomena in other media. The trans-' ducer element or elements employed may be either of the reversible type,- such as piezoelecof-two transducing elements workingon disslmtric crystal, moving coil, moving, armature. or

ilar components of the sound wave. Another object is to provide a unidirectional transdgcer with marked unidirectional properties over newsman; range of frequencies.

con enser type, or of the. non-reversible type such as for example, the carbon type. The theory set forth herein is applicable to receivingapparatus, suchas loudspeakers, as well.as

' A further object is to obtain an instrument to transmitting apparatus such as microphones.

If transducers of the reversible type are employed, one instrument could serve interchangeably, both as a transmitter and as a receiver.

The nature of my invention is such that it can be best explained by reference to the following eq'uivalent electrical network and circuit 'equations. Fig. '1 is a schematic representation of two electroacoustic transducers A and A, generating respectively voltages E and E', and the interconnecting electrical network 0. The transducers, which may operate on any function of the sound wave whatsoever, are spaced by an effective acoustical distance d which in general should be smaller than, or comparable. to, onequarter wavelength of the highest frequency at which unidirectional action it will be shown later that transducers may be constructed having unidirectional properties at frequencies higher than that specified above by virtue of diffraction and other wave'eflect, C is a generalized network shown in an equivalent or section, composed of impedances Z0, Z1, Z2, and Z, Z1, and Zn. The impedance Z: is connected to the receiver B which may be an or any other receiving device. For simplicity, the internal impedances of the transducers A and A are here considered negligible, although if this is not the case the proper internal impedances should be inserted in the network in carrying out the analysis.

The sound wave is considered as a plane wave which may be incident from any angle 0' from the normal 0 incidenceindicated with the corresponding arrow in Fig. 1. The voltage developed by the transducers A and A' is indicated as E and E respectively. Subscripts 0, 0 and 180 are used to designate voltages developed from any angle of incidence 0, for normal front (0") or for the rear (180") incidence of sound, respectively.- the transducers A and A will be displaced in phase by an angle given by the equation:

Y cos0 inwhich o. is the phase angle between the voltages E and E u is the expression 211 j is the frequency, cycles per' second 9 is the angle of incidence of sound C. is the velocity'of the sound wave Applying circuit analysis to the equivalent circuit of Fig. 1, it may be shown that the voltage 6 delivered to the receiving apparatus is given by the equation:

It may be shown, furthermore, drop across any branch in a network composed of linear elements, due to the action of two sources A and B connected at anytwo points,

is desired, although 7 The respective voltages generated by clarified by the following explanation made in- It is seen therefore that the expression for the voltage delivered to the receiving apparatus can be indicated in the form: 1

e=PE+QE' (n1) Any expression involving network elements,

having the function of P and Q in Equation III, 3

is herein called the network factor. v

To obtain unidirectional action, the voltage em should become zero. Therefore, the condition to be met is:

and hence the relation between coeiiicients P andQshouldbe such that:

and the nature of the network components is be chosen to substantially maintain this relation throughout the frequency range in which the unidirectional action is desired. 7

Equation V is perfectly general and may be applied to any unidirectional transducing system of my invention, its operation will be further reference to Figs. 2 and 3, which are vector diagrams representing the voltage relations for front and rear incidence of sound upon the instrument of Fig. 1.

For the purpose of explanation, it is assumed that voltages E and E, generated by the similar generators A and A, are of unequal magnitudes, although tlziis is not necessarily the case. The voltage E0 is shown leading the voltage E's through an angle is determined by Equation 1,

while the voltage Em is shown lagging behind Efm by the same angle, since reversal of the direction ofincidence brings about reversal of the relative phase positions of the generated voltages. The network factors P and -Q are shown of the same relative magnitudes and angular position as the rear incidence voltages Em and Em respectively, as specified by Equation V. The 0 (front-incidence) condition is shown in Fig. 2. The voltage E0 is operated upon the vector P to give the vector PEo which is the contribution of the generator A to the total output voltage. The voltage E's is operated upon by vector -Q giving the vector -QEo which is the contribution of the generator A to the total output voltage. QE'o is added to PEQ giving the resultant output voltage co.

The (180) rear incidence condition is shown in Fig.8. The voltage Em is operated upon by the vector P giving the vector PEm which is the contribution of the generator A to the total output voltage. The voltage E'm 'is operated upon by the vector -Q giving the vector QE'm which is the contribution of the generator A to the total output voltage. It should' be noticed that for the rear incidence condition, the voltages PEuo. and Ql 'ao are out of phase,

ship is:

. sine functions: hence, as long as:

asoasov be given in reference to Fig. 4. This network is the same as that of Fig. 1, with the following element values:

Z' a co 1 may; (v11) It will be assumed here that the voltages E and E are two vectors of equal magnitude and displaced by an angle whose value is deterto approximately equal the algebraic sum of the mined from Equation I, their ratio being therefore equal to a unit vector at the angle o. Subatituting this value of angle into the right hand side of Equation VI, and that of the network elements of VII into the left hand side of .Equation VI, it is evident that the desired relationlwL'C+jwC'R" ed represents a quotient of two vectors, each of which may be made very nearly a vector operating at an angle proportional to frequency, if

the relationship between resistance, inductance};

and capacitance is such that: v

=% 'and L= (IX) gives the following relation:

since substituting these values into Equation Di 1 The quantity in brackets of Equation XVI repre- It shouldbe observed that the numerator and the denominator of Equation X are the major terms of the expansion for the cosine and the -c'1z' 1 and -cn 1 The Equation x may be rewritten j sin'm(CR' 01? w c'1z' -o1z (x11 v The frequency term 0 drops onto! this equation, and therefore the condition for unidirectivitywillbeobtainedit %=R'C".RC' x111 The distance-d and'the velocity of sound 0'' being known, B, R, C, and C may be selected by the use of Equation XIII. Then values of L and I. may be computed from Equation 1x. Since the-Equation XIIholds as long as the expressions XI are true, then by choice of sufllciently small distance d, unidirectional action'may-be obtained throughout a wide range of frequencies. I have found that Equation xn isvalid up to frequencies at which (I is not larger than one-quarter the wavelength of sound; thus, if d is equal to aptained for all frequencies up to approximately 5,000 cycles per second.

The type of polar dlrectivity pattern obtained with the use of my invention depends upon the operational principle of the transducers A and A. This may be shown by solving the Equation V Q and substituting into Equation III, which gives:

m I E 190 I e-P(E E' =PE' Since A and A are similar generators, the ratio I of voltages E and E will be a vector K having constant magnitude and acting at the angle o therefore, the ratio of Eiao and E'io will be a vector K at an angle qbiao; therefore;

8 Irin 2 I -IL i The expression in parenthesis of Equation XV,

at frequencies for which it is small-compared to one-quarter wavelength of sound, may be shown angles o. and man. Substituting the values of these angles given by Equation I.

such. that the voltage generated 'is independent of v the incidence of sound (pressure-operated or nondirectional transducers), the polar characteristic of the combination will be a cardioid of revolution expressed by the quantity in parenthesis in Equation XVI. This polar characteristic is shown graphically in solid line in Figs 5. 5 v

If the voltage E and E varies as the cosine of the angle of incidence, which will occur if transducers are of the bidirectional or velocity-type, ,then E'=E'o cos (rand:

1 combining two velocity-type transducers and the network described resultsin an electroacoustic transducing instrument of very marked unidirectional properties. It will be observed that my invention may make use of any'two transducers proximately 1.5 cm., unidirectional action is 0b operating on the same wave function, even if their transducing principles were dissimilar.

Instead of providing the electrical network directly at the output of the transducers, it is possible to first amplify these outputs with two independent ampliflers and combine the outputs after the amplification. These procedure would be considered of the nature of an equivalent.

1 Instead of employing two transducers andan electrical networkv to obtain unidirectional operation, my invention makes such Operation possible through modifying wave disturbances at two points in space by means of equivalent acoustical network and impressing these disturbances upon one electroacoustical transducer. 'I'hus, the;

near is Sensitive to one pressure component 05 of, a wave emanating from some source in the me- 'dium, which component acts through one of such points and is sensitive also to another pressure component 'of the wave, this latter component acting through the other of such points. An embodiment of my invention employing this alterna- 7 tive is shown in cross section and rear elevation in 188; 6 and 7. The transducer The forcesdeveloped by sound pressure at the for assemblyw consists... in a diagram 22 suitably supported in a casing .which also contains the piezoelectric crystals 24..

4- diaphragm are transmitted to the crystal by means of a connecting member 25, and the electrical energy developed therein is received from the crystal by means of conductors 4i and 48. The front side of the diaphragm is provided with an acoustical damping screen 26 constituted of a suitable wire-screen support having one or more thicknesses of cloth forming acoustical resistance and inertance. Between the diaphragm 22 and screen 26, there is a cavity 21 having an acoustical compliance C.

The casing 23 has a circular opening 28 which serves as a housing for the piezoelectric crystal and also forms part of the acoustical network. The housing 23, the back plate 29, and the diaphragm 22 provide a. cavity 40. side of the case by means of screws II is held a cover 29 provided w th spacing being obtained by adjustment against compression of spring 42. Thus, a narrow passage 3| is .formed, having acoustical resistance and an inertance. P denotes the sound pressure at the outside of the damping screen; P denotes the sound pressure .at the outside of .the passage 3|. The effective acoustical path between tliese .pressures is called d. I have found that, at frequencies of sound for which the diameter of the casing 23 issmaller than one-half wavelength, the pressures' P and. P' are essentially equal and separated by a phase angle given by Equation I. y

The equivalent electrical circuit of the transducer and its associated acoustical network appears in Fig. 8 in which R and L, and R and L are the acoustical resistances and inertances of the screen 26 and the passage 3|, respectively; C and C are acoustical compliances of the cavities 21 and 40 respectively. Z: is the impedance of the transducer element itself. As a simplifying assumption, the impedance Z3 is considered as formed by the capacitance Ca correspondin to the stiffness of the crystal 24, and the reactionsof the medium are neglected. The volt-- At the rear pressure 0.11

ridge 30, the proper age e developed across Z: representsjthe resultant pressure upon the piezoelectric crystal. It may be observed that this equivalent circuit is entirely identical with that of Fig. 4, when the impedance Z: of Fig. 8 is perfectly general as in Fig. 4. The compleximpedance Z: in both flgures includes the special case of a capacitance C: as shown explicitly in Fig. 8.

In the commonly used system of electro-acous-.

tic analogies pressure, volume velocity, inertance, acoustic capacitance and resistance are considered the analogues of voltage, current, inductance, capacitance and resistance respectively.

This is because these acoustical quantities appear in the analysis of the behavior of acoustical circuits in precisely the'way the corresponding electrical quantities appear in the analysis of'anal-.

'ogous electrical circuits. The generalized circuit of Fig. 1 may therefore conveniently be used to illustrate the principle of operation of my invention in the special case in which a single transducer is used with acoustic means for modifying wave disturbances at two points in space to obtain unidirectional operation. In, this case E and E represent the efiective sound pressures at the input to the two acoustic network branches separated by an effective acoustical distance d.v

In' Figure 8 for example, E and E represent the pressures p and p of Fig. 6. The generalized network C is then an acoustical one and Z: is the effective acoustic impedance of the transducer, The voltage 6 represents the effective LII tions IX and XIII to provide the unidirectional action desired. The terms R, L, and Q due to screen 26 and cavity 21 are small compared with terms R, L and C due' to passage 31 and cavity 40, hence-the last term of the right hand side of Equation XIII will not have a great bearin upon the unidirectional action of the microphone. I have found that in some cases it is convenient to leave the damping screen 26 out altogether, and. when this.'1s done the constants R and C of the EquatiouXiII have toxbe readjusted slightly to compensate for disappearance of the last term. I have found that in a microphone with the casing '23 having a diameter of 6 cm. and the cavity having a volume of 8 cc., the effective distance d is 3.5 cm. and satisfactory operation is obtained when the passage 31 has a circumferential length of 10 cm., a radial length of 0.1 cm. and a thickness of 0.01 cm. These dimensions give an approximate acoustic capacitance C of 537x10 cm. per-dyne and an approximate acoustic resistance pfR' of 18 acoustical ohms. Since C, R and d are not calculable with good degree of accuracy in terms of the physical dimensions of "the instrument, I prefer to calculate the approximate dimensions for these terms, and obtain the final values by adjusting the thickness of the passage II by means of the screw 4| until correct unidirectional action is obtained. Obviously an alternative procedure would be to adjust instead the volume of the cavity 40 or the length of the distance d which could be done by provisions for adjustably altering the size of the case 23.

The adjustment of the thickness of the passage 3! has a marked effect on the directional prop- 'erty of the microphone at low frequencies and therefore the provision of screws 4| to permit variations in this adjustment is desirable. In the specific structure described above acardioid unidirectional characteristic is obtained with a passage thickness of only 0.01 cm. When this passage is closed the sound wave has access only J (XVIII) in which 21. is the viscosity coeflicient of the medium I is the passage length in cms. in the direction of flow t is thepassage thickness in cms.

L is the peripheral length of the passage p is the density of the medium in grams per cubic centimeter, and

w is 21 times the frequency.

From this we note that the acoustic resistance varies inversely as the cube and the inertance inversely as'the passage thickness. At low frequencies the reactance of the inertance is small which results from the time required for the sound wave to travel the distance d. The right.

hand side w(C"R'CR) represents the difierence between the phase shift the pressure experiences in passing through the rear-acoustical network to the diaphragm and the phase shift the pressure p experiences impassing through the front acoustical network to the diaphragm. If we define the quantity is as (C'R'CR)C'v/d that is as the ratio of this diflerence to thephase difference between E and E then the resulting response of the microphone, r, in polar coordinates, in terms of the angle of sound incidence, t, is given by r=p(kcos 6) where p is proportional to the maximum pressure of tin: sound wave. This is the equation of-the limacon. By substituting the values of R given by Equation XVIII in the expression for k and using this value of k in the equation of the limacon the variation in, directional response with-passage thickness may be determined. The response is zero for the values of which make cos 0=--7c.

When It equals one, that is when Equation XIII- holds, a cardioid directional pattern results,, '1'l1e value of cos 0 then equals -1 when ,0 equals.

180 and there is no response from' the rear of the microphone: This is the directional jcharac-. teristic usually desired and the one to which I have given detailed consideration above. A plot ofsome of the other directional characteristics the microphone is capable of giving are shown in Figure 22. As It is made less than one, the mi-- crophone maintains its unidirectional, property, or preferentialiresponse to sound of 0 incidence, but a minor'lobe corresponding to diminished re sponse to sound of rear incidence occurs. The

. two angles of zero response, which may be thought of as being coincident for the special case of k equals one, are symmetrically disposed with respect to 180 in the second. and third quadrants. In the limit when k equals zero, that is when R'C' equals R0 the null angles are 90 and 270 and the microphone becomes a cosine bidirectional type. 4

The polar pattern shown in Figure 22 represents the response of the instrument in a, plane through the principal axis of the microphone. The surface of revolutiongenerated by rotating the curve about this axis represents the three dimensional response of the device. From this we note that when k equals '1, the response is zero on the principal axis for sound of 180incidence.

-As It is reduced the surface in which zero response occurs is a conical one, the internal solid angle of which increases as k is reduced until in the limit when k equals zero the surface becomes the plane of zero response in the resulting cosine bi-- directional microphone. As the valueof k is made to exceed one,the surface'of zero responsev disappears although themicrophone retains its unidirectional property" until the limiting value,

pa small fraction of the resistance R, of the passage, 30, so that the-sound wave has relatively free access to the front of the diaphragm. All

unidirectional characteristics of the microphone between the nondirectional and the cosine bi-- directional which are given by the equation for the limacon are obtained by varying k between zero and infinity. The value of infinity is obtained when the passage is closed. The value of zero is obtained when R'C' equals RC. Since the passage resistance varies inversely as the cube ofthe passage thickness, this latter adjustment is I obtained with a. thin passage even with low screen resistance. With a typical screen resistance in a structure of the type shown in Figure 6 a-cardioid ing the opening 28, as shown more clearly in Fig..

characteristic is obtained with a passage thickness of 0.01 cm.-and a cosine bidirectional characteristic with a passage thickness of the order of 0.03 cm. so that all unidirectional characteristics are obtained by changing the passage thickness from zero to 0.03 cm. Because all of the desired directional properties are obtainable witha very thin passage, 30, if the screen, 26, is properly' chosen, the left hand side of Equation VIII may be made proportional to frequency over a relatively wide frequency range even when the passage thickness is altered to giv diflerent directional characteristics.

Instead. of obtaining'resistance R by means of the passage 3|, it is possible to substitute the cover]! with a suitable foraminous supporting member such as a wire-screen disc havlnga number of thicknesses of cloth or felt or similar porous materialattached to it, completely cover- 13. The cloth illustrated is designated by the numeral 10. Fig. 13 is similar to Fig. 6 except i as to the use of the cloth screen 10 in place of the narrow passage 3|. By a suitable choice of the thickness and porosity of the material employed, the proper value of acoustic impedance 'may beobtained. Sometimes it is difficult to? select a material having the exact ratio of resistance to inertance specified in Equation IX; however, it is seen from Equation X that the squared terms are second-order terms in .e sion for cosine function, and therefore the exact relationship between the inertance term L and the resistance and capacitance terms R and C is not a vital one in obtaining the unidirectional operation of the instrument at low frequencies,

of infinity is reachedat which the'microphone' becomes nondirection Since :1 depends on the external shape of the microphone, as discussed above,'and Rand Care determined by the screen 26, and the volume of is varied by adjusting screws 4| which alter the and reasonable departure therefrom will aifect the unidirectional action but slightly. The important adjustment, however, is the one between the terms expressed in Equation m.

I have mentioned previouslythat the Equation XIII is valid up to frequencies at which (i is not larger than I approximately one-quarter wavelength of sound. strument of mately 2500 cycles per second. It should not be assumed, however, that above said frequency the unidirectional action ceases, because above 2500 cycles per second, the instrument tends to become highly unidirectional in favor of sounds arriving from the front because of diifractlon and the sovcalled baiiie eifect due to the size of the case 23.- The unidirectional action is therefore obtained essentially throughout all of the important frequency range.

I have found that when a'plane wave of con-- stant intensity and varying frequency is' inf- Thiscorresponds, for the in- 6, to a frequency of approxicardioid shown in solid lines in Fig. 5.

' shoe 55.

air in the passage 82.

pressed upon the front side of the instrument of T Fig. 6, the resulting alternating force upon'the crystal 24 isapproximately proportional to frequency up to the frequency at which one-quarter wavelength equals the effective distance d, be-

coming approximately independent of frequency crystal, the receiver 49 being connected across the larger condenser.

Fig. 9 shows the frequency response obtained with this microphone and electrical network for plane wave incident upon the front (upper curve) and the rear (lower curve) of the instrument, indicating the type of discrimination obtained at all frequencies. The polar directivity pattern is a It may be found convenient in many instances to provide the desired electrical compensation in the receiver 49. For applications in which'it is desired to give predominance to higher frequencies of sound, the compensating network may be entirely dispensed with.

Another embodiment of my invention is shown in the part sectional elevation in Fig. 10 and front elevation. in Fig. 11. A moving coil consisting of 'a circular bobbin 50 having a winding and a dome-shaped diaphragm or cover 52 is arranged to move in an air gap 53 of a magnetic structure, thereby transforming its mechanical motions into electrical energy which is received from the winding 5! by means of conductors SI.

is acoustical impedance of the coil and its suspension; R' and L, the resistance and inertance terms of the passage 82; C- is the acoustic compliance of the cavity 63; Rb impedance of the passage 60; Cs, the acoustical compliance of the chamber 58; as in the preceding embodiment, the reactions of the medium are neglected. The effective acoustical distance be tween the pressures E and E will be called d, and for frequencies at which this distance is less than a quarter wavelength of sound, E and E may be moved. Comparing these 12, and the capacity The magnetic structure consists of a cylindrical permanent magnet 54 provided at one pole with an internal circular pole piece 55, and having an external pole piece 58 connected to the other pole of the magnet by means of several connectingrods 51 which provide enough cross sectional area to conduct the magnetic flux, and'do not appreciably interfere at the same time with'the free access of the sound waves to the inner pole Associated with the magnet structure, however not performing flux carrying functions, is a circular plate 59 which defines the cylindrical chamber 58 and also bymeans of shims ill forms a narrow passage 80 which establishes communication between this chamber and the volume within the moving coil.

meter of the bobbin, thus forming a narrow pasinto the cavity 63 which is desage 82 leading fined by the moving coil. The stiffness of the suspension Si is low so that the coil assembly is resonated at a low frequency, preferably in the neighborhood of 60 cycles per second. The resonant effect is not very pronounced, however, because of the dampin resulting from motion of The equivalent electrlcaf'circuit of the instrument placed in a sound wave is given in Fig. 12. E is the equivalent of sound pressure upon the front of the diaphragm 52. E is the equivalent of the pressure of thesound wave at the passage 82; Z: 7

considered equal in magnitude and displaced by an angle 4 given inEquation' 1.

Comparing structures illustrated in Fig. 6 and Fig. 10, it will be noticed that the latter is similar to. the former with the screen 26 removed, since the impedance of the narrow passage 3| in Fig. 6 corresponds to that of the passage 82 in Fig. 10, and the compliance of the cavity 40 .in Fig. 6 corresponds to thetotal compliance of the cavities 58 and II in Fig. 10. This similarity may be further seen by comparing the equivalent circuits of Fig. 8 and Fig. 12, the former with the impedances R, L, and C (corresponding to the damping screen 2! and cavity 21 in Fig. 6) re equivalent circuits, the series impedances R and L of Fig. 8 correspond with the impedances C of Fig. 8 corresponds to the total capacity of the condensers C- and Ch in Fig. 12. Therefore, Equation XIII may be used to determine the correct proportion between the resistance and compliance units in the structure of Fig. 10, the last-term of the right hand member of the equation being set equal to zero since the front screen is not used in this embodiment.

However, in the'moving coil structure of Fig. 10, I

it. is not feasible to make the passage 2 narrow enough'to obtain the correct ratio between resistance and inertance as expressed in Equation 1:: and this would affect adversely the unidirectionalproperties of the microphone in the middle range of audio frequencies. I have found that by subdividing the compliance elements into two approximately equal parts shown as Ca and Cs interconnected by a series impedance having a resistance value Rs approximately three times the value of R and an inertance value It approximately equal throughout substantially all of the audio-frequency range.

I have found that in a microphone as. illustrated in Fig. 10 having an external'pole piece square and a moving passage 62 has an axial length of 0.16 cm. and a.

These dimensions givethickness of 0.007 cm. an approximate acoustivyalue of C. and Cs of 2.1 10- cm. per dyne each, R and Rs of 17.3 and 50 acoustical ohms respectively, L and Ls of .0015 gram per cm These termsare not calculable with high degree of accuracy and minor adjustments are required to obtain satisfactory operation which is similar to that indicated by the performance curves of Fig. 9. It will be understood that through judicious application of previously given theory and equations, the above dimensions may be considerably altered without departing from the scope of my invention.

A still further embodiment of my invention e pl'oying. a moving conductor as a .transducing element is shown in front elevation in Fig. 14.

and La, the acoustic of the same calling in Fig.

to L, the unidirectivity Equation VI isvery closely satisfied The voltage generating element is a light metalli'c conductor or ribbon I which may be corru-' gated to increase its flexibility, supported at its two ends on insulating supporting members I01 and adapted to move between pole pieces IOI of a magnetic structure and convert these motions into electrical energy which is received by means of conductors H2 and H3. The ribbon is almost as wide as the space between the pole pieces, being separated therefrom only enough to move freely therebetween. Figs. and 16 which are cross sectional views of the instrument along the lines I5 and I6 in Fig. 14, respectively, show that the pole pieces IOI and the ribbon I00 form a cavity I08 which is inclosed at the rear by a plate I04 suitably attached to the supporting member I01 by means of screws H5. The cavity I08 is in communication with the exterior by means of passages I06 formed between the pole pieces I M and the back plate I04, the width of said passages being determined by suitably adjusting the position of the supporting members I01 behind the pole pieces I0 I. The passages I06 have such proportions as to constitute essentially an acoustical mass element. 'The cavity I08 is provided with a parallel dissipative element formed by a pipe or conduit I09 of suitable cross section and length fitted at the near end into the back plate I04 and closed at the far end. The conduit is filled with dissipative material I I0, such as loose range of validity of Equations XIX and xx without departing from the spirit of my invention.

Figs. 18, 19, and represent a further embodiment of the invention which is somewhat along the same lines as that shown in Figs. 14 to 17.

- forms with the pole pieces I22 passages I30 1y packed felt, wool, or cotton, which is retained at the near end with a wire screen I II or similar foraminous retaining member.

It will be evident, however, that other suitable means may be used to produce the parallel resistance effect. The sound pressure at the front or exposed part of the ribbon will be called P and that at the entrance of the passage I06 will be called P. The effective acoustical distance between these pressures will be called d.

The equivalent electrical circuit of this microphone is shown in Fig. 1'7. E and E represent the sound pressures P and P respectively, L3

represents the acoustical impedance of the ribbon which is assumed to be a mass, and e is the force developed across the ribbon due to E and E, L1

is the mass of the passage I06, C2 is the compliance of the cavity I08, and R2 is the resistance oLthe conduit I09. Equation II may be applied to this circuit by substituting the appropriate value of the impedances. Following a reasoning similar to that employed with Equations IX to XII, it may be shown that unidirectional action-is obtained if and, in addition,

l 1 CTR,

description having an equivalent front-to-back distance d of 2 cm. satisfactory operation is obtained if the volume of the cavity I06 is 1.2 cm.,

' total length of the slit I06 is 10 cm., its transstants may be considerably'altered within the I have found that in a microphone of this which communicate at one end with the cavity I25 and at their other end with the atmosphere.

The thickness of passages I may be adjusted by means of the screws I29 as may be desired, for regulating the polar pattern of the device,

The back plate I26z-is1widened somewhat to permit the attachment of the shell or inclosure I3I by screws I32. The interior of inclosure I3I defines a cavity I33; Cavity I33 is acoustically connected with cavity I25, and consequently with the body I20 by the passages I34, the thickness of which is controlled by the position of the strip I35. Strip I35 is attached by means of spacers I36 and screws I31. The spacers may be resilient if desired to permit adjustment of the size of passages I34 through adjustment by screws I31.

,The cavities I25 and I33, and the passages I34 and I30 form an effective acoustical impedance operative on that part of'the sound which gains acoess'to the rear side of the ribbon body I 20.

The waves having access to the front side of rib-' bon' I 20 also are operated upon by an-acoustical network in the instant embodiment. This front network, though desirable, is not essential to the functioning of the device. ,As here shown, the front network comprises a cavity I39 formed by the front side of the ribbon I20 and the pole pieces I22. Also forming a part of the front network is the clamping screen I40 formed of a supporting screen I401: and a layer of cloth or foraminous material I40b. l

Comparing the embodiment of Figs. 18, 19 and 20 with the embodiment of Figs. 14, 15 and 16-.

\ the cavity 'I25,corresponds with cavity I08 and the combined impedance of passage I34 and cavity I33 corresponds with the combined impedance of screen I II and filled conduit I09, the impedance of both of these combinations being predominantly resistive over a substantial frequency range. The modification shown in Figs. 14 to 16 has no front acoustical network corresponding with cavity I39 and screen I40 of Figs. 18 to 20, and when only.

moderate adjustment of unidirectional characteristics is desired the screen I40 may be dispensed with, leaving a network only at the rear of the device. I

The equivalent electrical network for this embodiment is given in Fig. 21 where the combined inertance and resistance of the screen I40 is represented by R and L. The acoustic capacitance of the cavity I39is represented by C. Z: represents the acoustic impedance of the body I20, and will usually constitute principally inertance.

The acoustical capacitance of the cavity I25 including also the capacitance oi passages I34 and I30 is represented by Ca in Fig. 21. The

The electrical varia-.

than passages 30 diaphragm phragm I is moved near the rear plane of the impedance of the passage is predominantly re-.

sistive at low frequencies. If then the cavity I33 has adequate volume the reactance of Cb may be neglected over an appreciable frequency range so that the principal effect of the combined passages I34 and cavity I33 is resistive.

The inertance of passages I30 is represented on the equivalent electrical diagram by Li. In general the thickness of the passage I30 is such that the resistance is small in comparison with the inertance so that the latter predominates as was true in connection with the corresponding passages J06 of Figs. 14 to 17. For this reason the passages I30 are normally appreciably thicker in Fig. 6 which is designed to be predominantly resistive.

I have found it possible to obtain desirable directional characteristics when the combined area of passages I30 is comparable to the area of the I20, and this is true even when diapole pieces I22. Though the volume of cavity I is so reduced its effect is retained by the effective volume of the inner ends of the enlarged passages I30.

The specific values of the elements 0., Rb, Ls, Cb, and L1 are chosen to balance the front network formed by R, L and C, so that the sound waves which have access to the rear of the movable element I20 are shifted in phase to a greater extent than the waves having access to the front of the element I20, and so that this diflerence in the phase shift bears a constant ratio, over an appreciable frequency range, to the phase change across the effective acoustical distance between front and rear of the device. Usually these values are chosen so that the magnitude of the resulting pressure on the'rear of the device approximately equals'the magnitude of the pressure on the front of the diaphragm.

The approximate relation and 01 may be-obtained from Equations XX since these elements correspond to C2, B2, and d before treated.

Adjustment of the directional characteristics is most conveniently obtained by adjusting screws I29 thereby compressing or permitting expansion of spacers I21 and I20. Directional characteristics may also be changed by adjusting screws I31 to narrow or widen the passages I3l although the adjustment first mentioned is in the present structure more convenient, If desired, the structure could be varied to extend strips I" through an elastic seal in between Ca, Rb, L1 XIX and Li the inclosure IiI to make screws I31 available exteriorly of this inclosure.

While I have shown slots I30 extending both ways toward the outside, this slot or passage might be. single and in any desired shape. In general this would involve enlarging 'the area of the single passage.

While I have shown and described in detail several modifications of my invention, it is recognized that the invention may be practiced in many other embodiments, and the foregoing descriptionsand explanations are not to be taken in any limiting sense.

I claim:

1. In a unidirectional electro-acoustic transducer operating in a wave transmitting medium, means having two pressure sensitive surfaces adapted to vibrate and translate its vibrations into electrical energy, one of said surfaces being substantially exposed to the medium, a structure forming a cavity in conjunction with the second of said surfaces, means associated with said cavity defining an acoustical absorptive element having a resistance characteristic, said structure having communicating means defining essentially acoustic inertance between said cavity and the medium, said communicating means bein located from said exposed surface at a distance such that the ratio of said distance to the wave velocity in the medium approximately equals the ratio of said inertance to said resistance.

2. In a microphone operating in a medium, a light moving body having pressure-sensitive sides and adapted to move and change its motions into electrical energy, a structure forming a cavity associated with one side of said body, a passage defining essentially acoustic inertance communicating with said cavity and the medium, and absorptive means forming essentially acoustic resistance associated in parallel, in an acoustic sense, to the compliance of said cavity.

3. In a sound translating device, a moving body adapted to vibrate and convert its vibrations into electrical variations, said body being sensitive to sound waves emitted from a sound source in an elastic medium, structure which for waves of normal front incidence adds an equivalent acoustical distance to one pressure component of said wavesto which said body is sensitive, and an acoustical network imparting to said component a phase shift bearing a constant ratio to the phase change which takes place due to said waves travelling said distance in said medium throughout a substantial range of frequencies, said network containing one element which is substantially acoustical resistance and another element which is adjustable and which is substantially acoustical'inertance.

4. In a sound translating device, a moving body which is. senstive to two components of sound waves emitted from a sound source in an elastic medium, structure separating access of said components to said body by an equivalent acoustical distance, said body being adapted to move in response to said waves and change its motions into electrical variations, and an acoustical network through which one of said components have access to said body and including a resistance acoustically connected to said moving body and an adjustable inertance connecting said resistance to said medium, said network imparting to said one component a phase shift which bears a constant ratio to the phase change taking place when said waves travel said distance over a substantial range of frequencies.

5.'In a sound translating device operative in an elastic medium, a light moving body having pressure sensitive areas and adapted to move and change its motions into electrical energy, a

structure forming a cavity associated with one of said areas, a passage defining essentially acoustic inertance communicating with said cavity and'the medium and a chamber containing dissipative material forming essentially acoustic resistance associated in parallel, in an acoustic sense, to the compliance of said cavity.

6. In a sound translating device operative in an elastic medium, a moving body having pres- 'sure sensitive areas and adapted to move and change its motions into electrical energy, and an acoustical network through which sound waves have access to one of said areas and which is eftical network through which sound waves have. access to one of said areas and which is effective fective toshift the phase of waves passing to said one area, said network including a compliance adjacent said body, a resistance in parallel with said compliance, and an inertance connect- V ing said compliance and resistance to the medium.

7. In a sound translating device operative in an elastic medium, a moving body having pressure sensitive areas and adapted to move and change its motions into electrical variations, a cavity adjacent one of said areas and having essentially acoustical compliance, a dissipative chamber communicating with said cavity and having essentially acoustical resistance in parallel with said compliance, and a passage communicating with said cavity and with said medium and having essentially acoustical inertance, said cavity chamber and passage forming an acoustical network effective to shift the phase of the waves to which saidone area is sensitive.

8. In a sound translating device operative in an elastic medium, a moving body having pressure sensitive areas and adapted to move and change its motions into electrical variations, a

cavity adjacent'one of said areas, a chamber and a relatively narrow passage connecting said cavity and said chamber, said chamber and passage representing principally acoustical resistance and a passage connecting said cavitywith 'said medium and representing principallyacoustical inertance, said cavity, chamber and passage forming a network effective to shift the phase of waves to which said one area is sensitive.

9. A device as defined in claim '8' in which said relatively narrow passage is adjustable as to width to thus vary the phase shift created by said network. 10. In a sound translatingdevice operative in an elasticmedium, a moving body having pressure sensitive areas and adapted to move and change its motions into electrical energy, and an acoustical network through which sound waves have access to one of said areas and which is effective to shift the phase of waves passing to said one area, saidnetwork comprising a cavity adjacent said body and representing principally acoustical compliance and a chamber being. connected in said network in parallel with said compliance by a passage which with said chamber represents principally acoustical resistance.

11. In a sound translating device operative in an elastic medium, a. moving body having pressure sensitive areas and adapted to move and change its motions into electrical energy, acousto shift the phase of waves passing to said one area, a second acoustical network through which sound waves have access to another of said areas and which is effective to shift the phase of waves passing to said other areafsaid last-mentioned network comprising a cavity adjacent said body and representing principally acoustical compliance and a chamber connected in said network in parallel with said compliance by a passage which with said chamber represents principally acoustical resistance, the difference in phase shift produced by said networks bearing a constant ratio to the phase change taking place when said waves travel the distance between the entrances to said networks over a substantial range of frequencies.

12. A device as set forth in claim 11 in which said constant ratio is unity. 13. In a sound translating device operative in an elastic medium, a moving body having pressure sensitive areas and adaptedto move and change its motions into electrical variations, an acoustical network through which sound waves have access to one of said areas and which is effective to shift the phaseof waves passing to said one area, a second acoustical network through which soundwaves have access to another of said areas and which is effective to shift the phase of waves passsing to said other area, said last-mentioned network comprising a cavity adjacent said body and representing substantially acoustical compliance, a chamber connected in parallel with said compliance by a passage which with said chamber represents principally acoustical resistance, and a passage representing substantially inertance and connecting said cavity .to said medium; the'diiference in phase shift produced by said networks bearing a constant ratio to the phase change taking place when said waves travel the distance between the entrances to said networks over a substantial range of frequencies.

14. A device as set forth in claim 13 in which said first-mentioned network comprises a screen and a cavity bordered on one side by said screen and on another side by said one area. a

15. A device as set forth in claim 13 including means for adjusting the width of said passage to vary the inertance of said last-mentioned network and thus alter said constant ratio.

BENJAMIN B. BAUER.

CERTIFICATE OF CORRECTION.- Patent No. 2,305,597; Deoember 22, 19!;2;

. BENJAMIN B. BAUER.

"It is hereby certified, that the name of the assignee in the above nunbored patent was erroneously described and specified as "3.1. Shure and Frances Shure, Trustees, doing business es Shure Brothers, apartner ship" whereas said name should have been described and specified es --S. N. Shure and Frances Sbure, Trustee, doing business asshure Brothers, a partner ship-, as shown by the record of assignments in this office; and that the said Letters Patent snould be read with this correctiontherein that the samemdy conform to the record of the case in the Patent Office; v

Signed and sealed this 6th day of July, A'. D. 19b5- Henry Van Arsdale, I (Seel) A'cting coimhissione'r or Patents. 

